Solvent-based can lacquers, containing an epoxy resin and a cross-linking resin such as an aminoplast or phenolic resin are well-known. After application and during the heat-cure the solvent evaporates, and this is causing increasing concern in connection with air pollution regulations.
Two methods have been explored to develop a curable water-borne epoxy resin system: first, the epoxy resin ester approach, and second, the emulsion or dispersion approach. In the ester approach the epoxy resin is esterified with fatty acids and a sufficient amount of free carboxyl groups is taken up in the molecule to ensure, after neutralization, solubility in water. A serious drawback of such technique is that the ester bonds can be hydrolyzed in the water, and this type can therefore have insufficient storage stability.
In the emulsion or dispersion approach the unmodified epoxy resin is emulsified in the water, with the aid of suitable surfactants. The shortcomings of this technique results in the presence of the surfactant, which in the cured coating, will diminish the protective properties, and the application of such coatings with high speed roller coaters is often defective.
Recently, a third method has been proposed. To avoid these defects the epoxy groups of an epoxy resin are first reacted with the aminohydrogen of para-amino benzoic acid and then the carboxyl functions are neutralized to obtain a water-thinnable resin which can be cross-linked by aminoplast resins at usual baking schedules. See, for example, U.S. Pat. Nos. 4,094,844; 4,098,744 and 4,119,609.
The relatively high price and limited availability of para-amino benzoic acid limits the use of these compositions. Also, the resinous product requires a separate neutralization step.